Electrodes for use in applications as mentioned in the introduction are used to prevent or at least to slow down corrosion processes on the surface of various metallic structures, exposed to an ionically conducting medium. They comprise an active current transferring surface, which is in electric contact with the medium and which transfers a direct current between that surface and the surface of the structure, through the medium.
The electrodes shall be designed and located in such a way that they provide a desired current distribution on the surface to be protected. In addition to requirements as to current and/or resistance, a major requirement is that they have a sufficiently long service life. Furthermore, they shall have a high operational reliability and be electrically safe.
Cathodic metal corrosion protection systems are used to prevent or at least to slow down corrosion processes on metallic structures, typically steel structures such as pipe lines and storage tanks, exposed to, in this context, a medium such as sea water and soil. The structure to be protected is made a part of an electric direct current (DC) circuit, comprising at least one electrode, which is the subject of this application. The DC current is distributed over the surface of the structure to be protected and will then give the desired effect of preventing or at least slowing down corrosion processes on the surface of the structure. Often a plurality of electrodes are used in order to improve the current distribution on the surface to be protected.
In cathodic corrosion protection systems, the electrode becomes an anode in the current circuit, that is, the current is transferred from the electrode to the structure. Basically, such systems can then operate either according to the impressed current (ICCP) principle or to the principle of a sacrificial anode.
The electrode can either be located at some distance from the structure to be protected or, in certain cases, on the surface of the structure. In both cases, the current influencing the corrosion processes is transferred between the electrode and the structure via the ionically conducting medium.
Systems operating according to the ICCP principle comprise a current source, connected between the electrode and at least one so-called drainage point at the structure via a cable. Several drainage points, spaced out on the structure, may be used. The output current of the current source is controlled in dependence on a measured voltage between the surface of the structure and a point in the medium to which it is exposed, so as to keep the protective current density at the surface at an optimum level, high enough to prevent or at least slow down the natural corrosion processes, but less than a level, where, in this case, excess hydrogen ions may appear on the structure, making the material of the structure brittle or destroy the coating (paint) on the structure. The active part of the electrode is typically made of materials with a low dissolution rate, such as silicon iron and magnetite, or titanium, coated with platinum or with mixed metal oxides.
For systems operating according to the principle of sacrificial anodes, the anode material has to be electro-positive in relation to the material of the structure to be protected. The protective current is then maintained by the electric potential difference between the two materials. Typically, such material as magnesium, zinc or aluminum are used for protection of steel structures. These materials have, however, usually higher dissolution rates than the above mentioned materials used in systems operating according to the ICCP principle. Electrodes for use as sacrificial anodes are installed either directly on the surface of the structure or at some distance from it, in which case they are connected to the structure via a feeder cable.
Typical applications of cathodic corrosion protection systems are for protection of pipelines, both against corrosion on their outer surfaces and, when they carry ionically conducting liquids, also against corrosion on their inner surfaces. Other typical application are for protection of the inner and outer surfaces of storage tanks, the inner surfaces of condensers and heat exchangers, and for armored cables, for power transmission as well as for communication purposes.
In anodic corrosion protection systems, the electrode becomes a cathode in the current circuit, that is, current is transferred from the structure to the electrode. Such systems usually operate in a way similar to the ICCP principle, however with reversed polarity of the current source. After that a first stage of operation is completed, during which a protective passivation layer is build up on the structure, the current transferred by the electrode will drop to a lower level, high enough to maintain the layer on the structure. Anodic protection systems have a more restricted use than cathodic protection systems, typically they are used for protection of structures immersed in particular media, such as for instance protection of highly alloyed steels in an acid environment with an electrode made of copper. Such applications for anodic protection systems are for example in storage tanks, in heat exchangers and in the pulp and paper industry.
FIG. 1 illustrates schematically an electrical configuration typical for a known system for cathodic corrosion protection of the outer surface of a pipeline, operating according to the ICCP principle.
A power unit 60, supplied with electric power from an alternating current supply (not shown) delivers on its output terminals 601, 602 a DC current. The terminal 601 is, via a conductor 63, connected to the outer surface of a pipeline 61 (only a part of which is shown) at a drainage point 62, and, via a feeder cable 64, to a ground bed 15.
The ground bed comprises a plurality of electrodes 16, interconnection cables 2, interconnecting the electrodes among themselves, and a backfill (not indicated in the figure), for example coke, in which the electrodes are embedded. Each of the electrodes, which are made of for example silicon iron, are electrically connected to the feeder cable.
Usually, a plurality of systems such as described in connection with FIG. 1 are distributed along the pipe line, typically with an intermediate distance in the range of 10-15 km.
FIG. 2 shows a known alternative to the electrical configuration as illustrated in FIG. 1. The structure to be protected comprises a tank 65 and a piping system 66, located under ground. In this case, the electrodes 16 are not comprised in a ground bed but are located as discrete anodes at the tank and at the piping system. The electrodes are connected to the power unit via interconnection cables 2 and feeder cables 64,
FIG. 3 illustrates schematically an electrical configuration typical for a known system for cathodic corrosion protection of the outer surface of a pipe, operating according to the sacrificial anode principle. The structure to be protected is a pipe 65, located under ground. An electrode 16 is embedded in a backfill 67 and is connected to the pipe via a connection cable 68 at a drainage point 62.
FIGS. 4A-4C illustrate various prior art rod-shaped electrodes designed in an attempt to prolong their service lifetime, used for instance in corrosion protection systems operating according to the ICCP principle, for protection of the outer surfaces of pipelines and other structures. FIG. 4A shows an electrode 16 with two ends 101 and 102 and with a feeder cable 64 connected to the end 101. In the feeding end 101, the electrode has an increased diameter and is in addition protected by a sleeve 3, made of a non-conducting material. The active part of the surface of the electrode is in this case its total surface less that part of the total surface which is covered by the sleeve. FIG. 4B shows an electrode similar to the one as shown in FIG. 4A, the only difference being that it is provided with two feeder cables 64a, 64b, one at each end 101, 102 respective of the electrode, and with one sleeve 3 at each end. FIG. 4C shows a tubular electrode with a feeder cable connection 6 located at the center of it, the feeder cable connection being protected from incoming water by an insulation member 7.
FIG. 5 shows a prior art electrode for use as a sacrificial anode, for example in a configuration as illustrated in FIG. 3. The electrode is made of magnesium and comprises a steel insert 68a embedded in the electrode. At the feeding end 101 of the electrode, there is a recess 69 such that the cross section at the end 101 exhibits a groove in which the feeder cable 68 is connected to the steel insert via a feeder cable connection 68b (only indicated in the figure).
Typically, the active parts of the electrode are manufactured in the form of rods or tubular elements, which makes them easy to manufacture and to mount.
The surface of an electrode comprises an active part, which is in electric contact with the medium in which the electrode is embedded and through which the current is transferred, and often some part or parts covered with a non-conducting material, and which thus is/are not an active part of the surface of the electrode.
Dissolution of the material of the sacrificial anodes during their operation cannot be avoided and therefore an essential and basic problem with such electrodes is their limited service lifetime, which is typically much shorter than the lifetime of such structures to be protected. In order to prolong the lifetime of the electrodes, bigger sizes are often selected but still the electrodes must be replaced during their operation. Bigger anodes also have certain obvious disadvantages. For sacrificial anodes, the replacement is done typically three to five times during the lifetime of the protected structure, electrodes in systems operating according to the ICCP principle are typically renewed once or twice during that time.
For sacrificial anodes for protection of steel structures immersed in sea water, suitable electrode materials are magnesium, zinc or aluminum. A sacrificial anode of zinc or aluminum will typically lose 85 to 90% of its weight through electrolysis when the protective current to the protected structure is provided, the rest of the electrode being dissolved as a result of corrosion processes on the anode. For protection of structures embedded in soil, magnesium is typically used as electrode material, due to the lower specific conductivity of soil as compared to sea water, limiting the protective current. A sacrificial anode of magnesium has, however, a high self corrosion rate, typically up to 50%.
Another observed disadvantage of a sacrificial anode of the kind illustrated in FIG. 5 is that it starts to dissolve at the ends 101, 102, leaving the feeder cable connection and the steel insert exposed to the surrounding medium. This leads to a lower electrode voltage at the same time as the active surface of the electrode diminishes, resulting in a lower efficiency of the system.
To prolong the service lifetime of the electrode, various remedies have been proposed, as described above in connection with FIGS. 4A-4C. Thus, it has been proposed to increase the diameter of the electrode near the feeder cable connection. It has also been proposed to provide the end of the electrode with a sleeve of a non-conducting material. This measure, however, only moves the zone of dissolution to the edge of the coating. It has also been proposed to place the feeder connection at the mid of the electrode or to use two feeder connections, one at each end of the electrode. These measures have, however, only a limited effect on the service life of the electrode, achieved at the expense of more complicated and expensive designs.
Another disadvantage of such anodes which are installed directly on the structure to be protected, for use for example for protection of the inner surface of a pipeline, is that residual products, due to anodic and cathodic reaction processes, can be accumulated on the inner surface at the attachment point of the electrode, thereby making replacement of it difficult.